1. Field of the Invention
The present invention relates to a signal transmission structure, circuit board and connector assembly structure, and more specifically, to a signal transmission structure, circuit board and connector assembly structure, which can increase the impedance consistency of the signal transmission path.
2. Description of the Related Art
On the large-scale printed circuit board and packaging substrate, the signal lines used for the electrical connections between two components or two terminals are generally kept a consistent width or an approximate width, so that the characteristic impedance of the signal lines can remain the same. Especially for the signal transmission of high-speed and high-frequency signals, a better impedance match between two terminals is further required for the circuit designers because the un-matched impedance between the terminals will case the signal reflection and reduce the quality of signal transmission. More particularly, the insertion loss of the signal transmission should be reduced, and the return loss of the signal transmission should be relatively increased, so that the signal transmission quality could be maintained.
FIG. 1 illustrates a connection diagram of a conventional circuit board and an edge-feed SMA connector. FIG. 2 is a partial cross-sectional view of the circuit board in FIG. 1. FIG. 3A and FIG. 3B are side views of the SMA connector in FIG. 1 from the different viewpoints. FIG. 4A is a partial 3-dimensional view of the circuit board in FIG. 1, while FIG. 4B is the top view of the circuit board in FIG. 4A. With reference to FIG. 1, FIG. 2, FIG. 3A, FIG. 3B, FIG. 4A and FIG. 4B, FIG. 4A, there are only illustrated the conductive portion of the circuit board, while the insulation portion of the circuit board is omitted. The conventional signal transmission structure 100 is located at the edge of a circuit board 50, and the circuit board 50 is a 4-layer circuit board which is suitable for connection with an edge-feed SMA connector 70 through the signal transmission structure 100. The SMA connector 70 is a coaxial connector having a signal pin 72 and a plurality of supporting pins 74, and the supporting pins 74 are suitable for the mechanical connections between the circuit bard 50 and the SMA connector 70, like the supporting pins 74 are clipped the circuit board 50 for example. More specifically, the signal transmission structure 100 has a plurality of supporting pads 102, and the supporting pads 102 are electrically connected to a reference plane (e.g. a power plane or a ground plane) of the circuit board 50, for example through a plurality of through holes 131. The supporting pins 74 are connected with the supporting pads 102, and the supporting pins 74 clip the circuit board 50, so that the above SMA connector 70 is fixed on the circuit board 50, and is electrically connected to the reference plane of circuit board 50. In addition, the distance D between the supporting pins 74 shown in FIG. 3A is greater than or equal to the thickness of the circuit board 50 shown in FIG. 1, so that the supporting pins 74 can firmly clip the circuit board 50 after the supporting pins 74 are soldered on the corresponding supporting pads 102.
The circuit board 50 has four conductive layers, including the reference plane 110 inside the circuit board 50 and the conductive layer 120 on the surface of the circuit board 50. The conventional signal transmission structure 100 at least includes the reference plane 110 and the conductive layer 120. The reference plane 110 includes another portion of or whole conductive layer overlapping the conductive layer 120. In other words, the conductive layer 120 is disposed above one side of the reference plane 110. A dielectric layer 130 is further disposed between the reference plane 110 and the conductive layer 120 to insulate the reference plane 110 and the conductive layer 120 structurally and electrically. In addition, the conductive layer 120 includes a signal pad 122 and a signal line 124. The signal line 124 is connected with the signal pad 122, and the signal pad 122 is suitable for connecting with the signal pin 72 of the SMA connector 70 to transmit signals via the signal pad 122.
In the above signal transmission structure 100, the characteristic impedance value of the signal pad 122 is less than the characteristic impedance value of the signal line 124. The thinner dielectric layer 130, the more difference between the characteristic impedance values of signal pad 122 and the signal line 124. In other words, the impedance consistency of the signal pad 122 and the signal line 124 is not satisfactory, which causes increased insertion loss in signal transmission, thus adversely affecting the signal transmission quality.